将默认列值从日期时间转换为varchar

Question

I have to add a varchar column to a table which defaults to the current timestamp. To do this I somehow need to convert the value from datetime to varchar.

I tried the following

ALTER TABLE `TableName`
    CHANGE COLUMN `DocumentID` `DocumentID` VARCHAR(150) NULL DEFAULT CONVERT(NOW(), CHAR);

or

ALTER TABLE `TableName`
    CHANGE COLUMN `DocumentID` `DocumentID` VARCHAR(150) NULL DEFAULT CONVERT(CURRENT_TIMESTAMP, CHAR);

I always get an error message, that my syntax is wrong. I am using MariaDB and HeidiSQL. Is there a way to do this?

EDIT: I am basically looking for a MySQL equivalent of the following SQL Server statement:

ALTER TABLE [dbo].[TableName] ADD  CONSTRAINT [DF_TableName_DocumentID]  DEFAULT (getdate()) FOR [DocumentID]

Answer 1

MariaDB document says

In MariaDB 10.2.1 you can use most functions in DEFAULT. Expressions should have parentheses around them.

Hence you may check for the version of MariaDB and use the right syntax (parenthesis around expression):

 (CONVERT(CURRENT_TIMESTAMP, CHAR))

Update

As an alternative, you may use Trigger to set the function value for the stable releases (< 10.2)

Answer 2

If you use CURRENT_TIMESTAMP as default value, you must use the datatype that the function retrieves, in this case is a timestamp as you can see in the docs .

I don't know the reasons that may lead you to save this as a varchar. I'm not sure but I think that MariaDB doesn't allow to call functions as default values so you can't convert CURRENT_TIMESTAMP as varchar. You have an alternative approach, make an after insert trigger updating the field (you can call CONVERT(CURRENT_TIMESTAMP, VARCHAR) inside a trigger).

Either way, I recomend to stay with timestamp.

I see the field is called "DocumentID", perhaps you want to save a hash value as identifier? You can archieve this with virtual columns . An example .